Hydraulic brake boosters have been designed to provide an assist in the actuation of a master cylinder where a brake force is developed to effect a brake application. In order to reduce the cost of a hydraulic brake booster, often the same hydraulic power source used to supply a steering gear is used to power a hydraulic brake booster. The controls for such hydraulic brake boosters are designed such that a minimum amount of hydraulic fluid is always available for operation of either the hydraulic brake booster or the steering gear. In certain brake boosters of the type disclosed in U.S. Pat. Nos. 3,967,536; 4,131,055; 4,179,980; 4,514,981; 4,724,674 and 5,442,916, the booster operates by restricting flow from one side of a power piston to the other side of the power piston to create a fluid pressure differential which causes the power piston to move and provide power assisted displacement of the pistons in a master cylinder. In this type of brake booster, the master cylinder and booster are joined together and as a result the overall length occupies considerable under hood space of a vehicle. Because of the efficiency of such brake boosters they have found application in many vehicles and in particular van and certain mid-sized trucks. However, in recent vehicle models the physical design of the under hood space is often restricted or reduced, and consequently locating a brake booster and other components is often a difficult task. To better utilize under hood space, it has been suggested in U.S. Pat. Nos. 5,329,769, 5,313,796 and U.S. patent application Ser. No. 09/015,166, (U.S. Pat. No. 5,966,629) that certain brake systems components could be located remotely from under the hood. The control valve disclosed in U.S. patent application Ser. No. 09/015,166 (U.S. Pat. No. 5,966,629) functions in an adequate manner, however it is desirable to provide for a more stable or smoother application of the spool valve in the development of pressurized fluid to initiate a brake application.
U.S. patent application Ser. No. 09/097,778 (98/7530) filed Jun. 15, 1998 and now U.S. Pat. No. 6,038,857 discloses a brake system having a booster with a control apparatus wherein the rate of application of a spool valve is regulated to assure a smooth and effective development of a braking signal from a supply fluid derived from a pump to operate a remotely located master cylinder. In the absence of supply fluid, movement of an input member within the booster pressurizes fluid to provide a braking signal for operating the service brake and provide an emergency back-up brake for a vehicle. This brake system is adequate for certain applications but may not provide for sufficient back-up braking to meet proposed industry standards for future vehicles.